In a communication carrier system, a baseband signal is modulated with a carrier wave, and information in the baseband signal is extracted from the modulated wave received by the receiver. During the transmission progress, random offsets or noises are introduced to the carrier wave. As a result, the frequency of the carrier wave received by the transmitter and that received by the receiver are different from each other. If the difference between the carrier wave at the transmitter and that at the receiver is too large, such extraction is failed.